The invention relates to a workpiece, in particular to a rotor disc, to an impeller or to a guide wheel for a turbine, a pump or a compressor, and also to a welding method for the manufacture of a workpiece.
It is known to manufacture rotating rotor discs, pump wheels, impellers and also fixed guide rings for pumps, compressors or turbines from solid material by chip-forming machining. In rotor discs and guide wheels of simple geometrical construction, which are moreover not exposed to very high loads in the operating state, the rotor discs can also be assembled from individual components or made in one piece. This particularly applies to rotor discs of simple construction, which are not exposed to very large mechanical loads because they are not themselves moved and thus no imbalance or centrifugal forces occur. However, relatively slow running rotor discs can also be constructed from individual components with relatively little effort, for example, with suitable fits being provided on the components, which are then permanently connected by welding. However, these welds essentially only serve to fix the position and are not suitable for the transmission of larger forces and loads.
In this connection, the fixing of cover discs or hub discs to the blades of impellers is particularly problematic. This problem will be discussed briefly in the following by way of example and with reference to radial and axial compressors; however, the problem can in principle also be relevant to other corresponding apparatuses.
The radial compressor, for example, like the radial pump, consists essentially of a rotating rotor disc, also known as an impeller or a bladed disc, which is mounted on an axle and which can not only be open but also provided with a cover. In this special case the rotor disc can also be arranged in a surrounding fixed guide wheel, or enclosed by a spiral-shaped collecting space. In this arrangement the guide wheel has the shape of a diffuser in which a part of the kinetic energy produced in the rotor is transformed into pressure energy. In this embodiment, the guide wheel essentially comprises an upper disc and a lower disc, also termed cover or base, between which the guide vanes can be located.
The axial compressor including a rotor and a stator includes, in known manner, a rotor designed as a rotor disc with a drive shaft and guide vanes, and the rotor discs can be designed with or without an outer ring, in other words with or without a cover. The stator is formed as an enclosing housing in which the fixed guide vanes are accommodated, and a stator does not have to be provided in each case, such as in the case of certain ventilators for example.
It should be made clear at this point that, within the context of this application, the term “guide vane” is to be understood to mean not only the well-established term for the blades of a rotor disc but also the blades of a guide wheel.
In this arrangement, the rotor discs are heavily loaded in the operating state, since they are in part exposed to considerable centrifugal forces with enormous rotational speeds of up to 15,000 U/min at a diameter of the rotor disc of 400 m for example. In this arrangement circumferential speeds at the outer diameter of up to 400 m/s and more are absolutely within reach.
For this reason it is known to manufacture the rotor discs from solid material, depending on the application, for example from high-strength stainless steels, from super-alloys or other suitable metals or metal alloys, and to machine the guide vanes by means of a chip-forming machining process, by milling for example. When the rotors, such as in the case of an impeller, have to be additionally equipped with a cover part in the form of a cover, it is often no longer possible to mill the rotor disc as a whole in one piece out of solid material for purely geometrical reasons, but rather a one-part base body, which sits on the drive shaft of the compressor, is machined with guide vanes so that a cover part has to be subsequently placed on the guide vanes and has to be connected to them reliably. Alternatively, in special cases, it can also be possible for a cover part with guide vanes to be machined in one piece from solid material and for the base body, which is coupled to the drive shaft, to be subsequently assembled with the cover part with guide vanes and connected to form a complete impeller.
The problematic part is thus the connection of the guide vanes to an associated cover part or to a base body. In this connection DE 102 61 262 A1 discloses a method using guide vanes with cover discs which are connected by means of edgeless seams or penetration seams by means of laser welding or thermionic welding by butt welding or by through-going weld seams.
The disadvantage of this and related methods known from the prior art is, among other things, that notches in the material can lead to cracking forces so that unacceptably large notch effects can occur which, under the influence of the enormous centrifugal forces in the operating state, can lead to damage to the rotor disc which can lead ultimately to the destruction of the rotor disc and thus of the associated machine, for example of the compressor, of the pump or of the turbine.